1. Field of the Invention
The invention relates to a semiconductor device and a method for manufacturing same and, more particularly to, the semiconductor device provided with a capacitor in such a configuration that its capacity insulator film consists of a high-permittivity insulator film and a method for manufacturing the same.
The present application claims priority of Japanese Patent Application No. Hei 11-350894 filed on Dec. 9, 1999, which is hereby incorporated by reference.
2. Description of the Related Art
Large-Scale Integrations (LSIs) known as a representative of semiconductor devices are roughly divided into memory devices and logic devices, the former of which in particular has been developed remarkably by recent improvements in semiconductor manufacturing technologies. The memory devices are further divided into Dynamic Random Access Memories (DRAMs) and Static Random Access Memories (SRAMs), mostly all of which are comprised of Metal Oxide Semiconductor (MOS) transistors excellent in integration density. DRAMs, moreover, can enjoy the above-mentioned advantage of high integration density more than SRAMs, to reduce cost, thus finding a wide range of applications in a variety of memory units used in information equipment or a like.
Since DRAMs utilize each capacitor as an information storing capacitive element to store information based on existence/non-existence of charge held therein, with increasing storage capacity an area occupied by each capacitor becomes restricted which is formed on a semiconductor substrate. Therefore, the capacitors each need to have a larger capacitance. If the capacitance does not have a capacitance enough to hold information, an external noise or a like may cause malfunction readily, thus tending to yield an error represented by a software error.
To increase the above-mentioned capacitance of each capacitor, an insulating material employed as a capacity insulator film needs to have a large permittivity, such as tantalum oxide (Ta2O5), one of metal oxides widely used as a high-permittivity insulating material. The tantalum oxide film has approximately ten times the permittivity of a silicon oxide (SiO2) film conventionally used as the capacity insulator film and also approximately four times the permittivity (25 to 30) of a silicon nitride (Si3N4) film conventionally used as well. Therefore, the storage capacity can be increased by making the capacitors using tantalum films as the capacity insulator film.
Also, the capacitor having such a Metal Insulator Metal (MIM)-structure is widely adopted that, along with a tantalum oxide film used as the capacity insulator film, a titanium nitride (TiN) film excellent in step coverage is used to form upper and lower electrodes on opposite vertical surfaces of this capacity insulator film.
FIG. 11 is a cross-sectional view showing a configuration of a prior art semiconductor device having such a MIM-structure capacitor (hereinafter referred simply to as capacitor). As shown in FIG. 11, in an active region surrounded by an element isolating insulator film 52 formed for example on a P-type silicon substrate 51, a gate oxide film 53 and a gate electrode (word line) 54 are formed to thereby locally form an N-type diffused region 55 serving as a source or drain region, a surface of which is covered by an inter-layer insulator film 56 consisting of a silicon oxide film or a like. For simplicity in explanation, the N-type diffused regions 55, which are formed in pair, are illustrated as only one.
The inter-layer insulator film 56 on the N-type diffused region 55 has a contact hole 57 formed therein, which in turn has a capacitor 61 formed therein, which is connected to the N-type diffused region 55. The capacitor 61 comprises a lower electrode 58A consisting of a titanium nitride film, a capacity insulator film 59A consisting of a tantalum oxide film formed thereon, and an upper electrode 60A consisting of a titanium nitride formed thereon.
The following will describe a method for manufacturing the above semiconductor device along the following steps with reference to FIGS. 13A to 13D.
First, as shown in FIG. 13A, on the P-type silicon substrate 51, for example, the element isolating insulator film 52 made of silicon oxide is formed using a known method of LOCal Oxidation of Silicon (LOCOS) or a like and then, in an active region surrounded by this element isolating insulator film 52 are formed a silicon oxide film and a poly-silicon film in this order, which films are subsequently patterned into desired shapes to form the gate oxide film 53 and the gate electrode 54 respectively. Next, using the gate oxide film 53 and the gate electrode 54 as masks in self-alignment, an N-type impurity is introduced to the P-type silicon substrate 51 using a known impurity introducing method such as ion implantation, to locally form the N-type diffused region 55 constituting a source or drain region, following which the inter-layer insulator film 56 made of silicon oxide film or the like is formed throughout thereon using Chemical Vapor Deposition (CVD) or the like.
Next, as shown in FIG. 13B, the contact hole 57 is formed in the inter-layer insulator film 56 on the surface of the N-type diffused region 55 using photolithography and then, by use of CVD or the like, a titanium nitride film 58 which provides a lower electrode film is formed everywhere thereon. Next, as shown in FIG. 13D, using CVD or the like, a tantalum oxide film 59 which provides the capacity insulator film 59A is formed on the titanium nitride 58 in an atmosphere containing oxygen.
Next, In an oxidizing atmosphere consisting of a UV (Ultraviolet)-O3 (ozone atmosphere given by irradiation of ultraviolet light), the silicon substrate 51 is subject to heat treatment at about 500xc2x0 C. (annealing) to oxidize the tantalum oxide film 59, thereby improving film quality of the tantalum film 59 so that it can serve as the capacity insulator film 59A correctly. That is, the tantalum oxide film 59 has a problem in terms of a leakage characteristic if it is used as the capacity insulator film 59A, so that it must be oxidized to enhance its insulation performance, thus suppressing a leakage current.
Specifically, if, for example, this tantalum oxide film 59 is not sufficiently oxidized (that is, xxe2x89xa64 for Ta2Ox), it is to be further oxidized to a sufficient level to improve its quality, thus providing a stable film (that is, Ta2O5).
Next, as shown in FIG. 13D, on the tantalum film 59, a titanium nitride film 60 is formed which provides an upper electrode film 60A, using CVD or the like. Next, the titanium nitride film 58, the tantalum oxide film 59, and the titanium nitride film 60 are patterned using photolithography to form the upper electrode 60A, thus completing a semiconductor device having the capacitor 61 as shown in FIG. 11.
By the prior-art semiconductor device manufacturing method, however, the titanium nitride film 58 which provides the lower electrode 58A is readily oxidized and, in fact, is oxidized during the above-mentioned heat treatment of the silicon substrate in an oxidizing atmosphere, so that as shown in FIG. 12, a titanium oxide (TiO2) film 58B is formed on the surface of the lower electrode 58A consisting of the titanium nitride film 58. With this, this titanium oxide film 58B problematically acts as a low-permittivity film. If the titanium oxide film 58B having a low permittivity is thus formed on an interface of the lower electrode 58A and the capacity insulator film 59A, this titanium oxide film 58B is connected in series with the capacity insulator film 59A to act as part of that capacity insulator film 59A, so that total capacitance of the capacitor 61 decreases because it is affected by the low-permittivity film. Therefore, even if a tantalum oxide film which provides a high-permittivity film is employed, it is difficult to increase the capacitance of the relevant capacitor 61.
Permittivity of titanium oxide films is discussed in, for example, Japan Journal of Applied Physics (Vol. 38 (1999), pp. 6034-6038). This paper illustrates a relationship, as shown in FIG. 14, of the heat treatment temperature (horizontal axis) and the permittivity (vertical axis), which indicates that the permittivity of a titanium oxide film will change with the temperature of heat treatment. As an example, it indicates that the permittivity will be about 22 for heat treatment at about 600xc2x0 C. and decrease for heat treatment at lower temperatures.
By the above-mentioned heat treatment, the titanium oxide film 58B formed on the surface of the titanium nitride film 58 constituting the lower electrode 58A has a permittivity of about 15 or less, which is considerably lower than that of the tantalum oxide film 59 (25 to 30 as mentioned above).
The titanium oxide film 58B constituting a low-permittivity film, on the other hand, acts to decrease the total capacitance of the capacitor 61 but, as an advantage, serves as a leakage-current stopper film for suppressing leakage current of the capacitor 61. Therefore, if the titanium oxide film 58B is not formed at all, the leakage current tends to increase. In this case, however, the titanium oxide film 58B acts as part of the capacity insulator film 59A, so that the capacitance of the capacitor 61 decreases as its film thickness increases too much.
Note here that there is observed a difference in leakage-current characteristic of a completed capacitor between a case where the above-mentioned UV-O3 oxidation is performed at a relatively high temperature, that is, strong oxidation, and a case where it is performed at a relatively low temperature, that is, weak oxidation. FIG. 15 shows an example of the capacitor leakage-current characteristic, that is, a relationship of an application voltage (horizontal axis) and a leakage current (vertical axis), of a capacitor which has undergone strong oxidation, in processing, at about 500xc2x0 C. for about ten minutes and completed. FIG. 16, on the other hand, shows an example of the leakage-current characteristic of a capacitor which has undergone weak oxidation at about 400xc2x0 C. for about ten minutes and completed.
As apparent from comparison of FIGS. 15 and 16, the characteristic shown in FIG. 15 of the capacitor completed by strong oxidation indicates that a larger thickness is obtained of the titanium oxide film formed on the surface of the lower electrode and so the leakage current is reduced but, at the same time, its film thickness calculated as silicon oxide film, teq, has a relatively large value of about 3.2 nm, thus reducing the capacitance.
The characteristic shown in FIG. 16 of the capacitor completed by weak oxidation, on the other hand, indicates that the titanium oxide film formed on the surface of the lower electrode has a smaller film thickness and so has a larger capacitance but, at the same time, its film thickness calculated as silicon oxide film, teq, has a relatively small value of 2.5 nm, thus increasing the leakage current.
Note here that the film thickness, teq, calculated as silicon oxide film represents a film thickness calculated as being equivalent to the thickness of a silicon oxide film required to obtain a predetermined capacitance, indicating that the smaller the thickness, the better the performance.
Thus, although a low-permittivity film must be present which consists of the titanium oxide film 58B formed on the surface of the lower electrode 58A of the capacitor 61 when heat treatment is being performed for improving the quality of the tantalum oxide film 59 used as a capacity insulator film 59A, film thickness of this low-permittivity film provides a negative correlation between capacitance and leakage current of the capacitor 61 Japanese Patent Application Laid-open No. Hei 7-14992, for example, discloses a semiconductor device and a method for manufacturing a same for preventing a low-permittivity film from being formed on an interface of a lower electrode and a capacity insulator film and thereby implementing a capacitor having a large capacitance and a small leakage current. This semiconductor device comprises, as shown in FIG. 17, a capacitor 70 which includes a lower electrode 72 which has a thickness of about 100 nm and consists of a Ta (tantalum) film formed on a silicon substrate 71, a first capacity insulator film 73 which has a thickness of about 5 nm and consists of a tantalum oxide film formed on the lower electrode 72, a second capacity insulator film 74 which has a thickness of about 25 nm and consists of a titanium oxide formed on the first capacity insulator film 73, and an upper electrode 75 which has a thickness of about 100 nm and consists of a titanium nitride film formed on the second capacity insulator film 74. It is supposed that in this configuration of the capacitor 70, by constituting the capacity insulator film using the first capacity insulator film 73 (which is described to have a permittivity of 20 or larger) consisting of a tantalum oxide film and the second capacity insulator film 74 (which is described to have a permittivity of 100 or larger) consisting of a titanium oxide film, a low-permittivity film is prevented from being formed at an interface of the lower electrode 72 and the second capacity insulator film 74 and, at a same time, there is formed the first capacity insulator film 73 having a high permittivity, thus making implementing such the capacitor 70 that has a large capacitance with a reduced leakage current.
The following will describe a method for manufacturing the same semiconductor device with reference to FIGS. 18A and 18B. First, as shown in FIG. 18A, a tantalum film 72A and a titanium film 74A are formed on the silicon substrate 71 in this order by using a sputtering method. Next, as shown in FIG. 18B, the titanium film 74A is completely oxidized by plasma oxidation, to form a titanium oxide film 74B and, at the same time, the tantalum film 72A is oxidized only on its surface to form a tantalum oxide film 72B. Next, a titanium nitride film (not shown) is formed on the titanium oxide film 74B to subsequently pattern this titanium nitride film into a desired shape using an ordinary photolithography technique so as to form the upper electrode 75, thus completing the capacitor 70 shown in FIG. 17.
The prior art semiconductor device and method for manufacturing the same disclosed in the above publication, however, have a problem that a large capacitance cannot be obtained because a relevant titanium oxide film used as a capacity insulator film is difficult to act as a high-permittivity film.
That is, although the above publication says that a permittivity of 100 or larger can be given to the titanium oxide film 74B used as the second capacity insulator film 74 constituting the capacitor 70 such as shown in FIG. 17, the permittivity of the titanium oxide film 74B changes with temperature of heat treatment as shown in FIG. 14, so that it is difficult, as described in the above publication, to form a high-permittivity film by plasma oxidation.
Rather, it is much possible for the titanium oxide film formed by plasma oxidation to fluctuate in permittivity so that such a film having a low permittivity of about 15 or lower may be formed as the titanium oxide film 58B obtained by oxidizing the surface of the lower electrode 58A, as described with FIG. 12. Thus, if the permittivity of the capacity insulator film which determines the capacitance of the capacitor is liable to fluctuate, it is difficult to acquire a capacitance necessary for the capacitor to operate normally.
In view of the above, it is an object of the invention to provide a semiconductor device and a method for manufacturing same for suppressing a leakage current while securing a capacitance necessary for its capacitor to operate normally.
According to a first aspect of the present, there is provided a semiconductor device provided with a capacitor which is so formed as to be connected to one diffused region on a semiconductor substrate, the capacitor including:
a lower electrode consisting of a metal film which is so formed as to be connected to the diffused region;
a first capacity insulator film consisting of a high-permittivity insulator film formed on the lower electrode;
a second capacity insulator film which consists of a oxide film made of component metal of the lower electrode formed on an interface of the first capacity insulator film and the lower electrode, which has a lower permittivity than the first capacity insulator film, and which has a predetermined film thickness; and
an upper electrode consisting of a metal film formed on the first capacity insulator film.
Also, according to a second aspect of the present, there is provided a semiconductor device provided with a capacitor which is so formed as to be connected to one diffused region on a semiconductor substrate, the capacitor including:
a lower electrode consisting of a metal film which is so formed as to be connected to the diffused region;
a first capacity insulator film consisting of a high-permittivity insulator film formed on the lower electrode;
a second capacity insulator film which consists of a metal oxide film formed on the first capacity insulator film, which has a lower permittivity than the first capacity insulator film, and which has a predetermined film thickness; and
a metal film formed on the second capacity insulator film.
In the foregoing first or second aspect, a preferable mode is one wherein the lower electrode is connected through a capacity contact to the diffused region.
Also, a preferable mode is one wherein the lower electrode or the upper electrode is made of titanium nitride, titanium, tungsten nitride, or tungsten.
Furthermore, a preferable mode is one wherein the first capacity insulator film is made of tantalum oxide.
Still furthermore, a preferable mode is one wherein the second capacity insulator film is made of titanium oxide and has a film thickness of 0.2 to 1.0 nm.
Also, According to a third aspect, there is provided a method for manufacturing a semiconductor provided with a capacitor so formed as to be connected to one diffused region on a semiconductor substrate, including the steps of:
locally forming, as a diffused-region forming step, a second-conductivity type diffused region on a first-conductivity type semiconductor substrate;
forming, as a lower-electrode forming step, a lower electrode consisting of a metal film constituting the capacitor in such a manner that the lower electrode may be connected to the one diffused region;
sequentially forming, as a capacity insulator film forming step, a first capacity insulator film consisting of a high-permittivity insulator film constituting the capacitor on the lower electrode in a plurality of sub-steps;
performing heat treatment, as a semiconductor-substrate heat treating step, on the semiconductor substrate in an oxidizing atmosphere for each of the plurality of sub-steps of the capacity insulator film forming step, to thereby form a second capacity insulator film consisting of an oxide film made of component metal of the lower electrode on an interface of the first capacity insulator film and the lower electrode; and
forming, as an upper-electrode forming step, an upper electrode consisting of a metal film constituting the capacitor on the first capacity insulator film.
Furthermore, according to a fourth aspect of the present invention, there is provided a method for manufacturing a semiconductor device provided with a capacitor so formed as to be connected to one diffused region on a semiconductor substrate, including the steps of:
locally forming, as a diffused-region forming step, a second conductivity-type diffused region on a first conductivity -type semiconductor substrate;
forming, as a lower-electrode forming step, a lower electrode consisting of a metal film constituting the capacitor in such a manner that the lower electrode may be connected to the one diffused region;
forming, as a capacity insulator film forming step, a first capacity insulator film consisting of a high-permittivity insulator film constituting the capacitor on the lower electrode;
forming, as a metal film forming step, a metal film capable of forming a metal film on the first capacity insulator film;
performing heat treatment, as a semiconductor-substrate heat treating step, on the semiconductor substrate in an oxidizing atmosphere to oxidize the metal film, thus forming a second capacity insulator film consisting of a metal oxide film to a predetermined film thickness; and
forming, as an upper-electrode forming step, an upper electrode consisting of a metal film constituting the capacitor on the first capacity insulator film.
In the foregoing third or fourth aspect, a preferable is one that wherein further including a step of forming, a capacity contact forming step, a capacity contact in such a manner that the capacity contact may be connected to the diffused region, between the diffused-region forming step and the lower-electrode forming step.
Also, a preferable mode is one wherein as a material of the lower electrode or the upper electrode, titanium nitride, titanium, tungsten nitride, or tungsten is employed.
Also, a preferable mode is one wherein as a material of the first capacity insulator film, tantalum oxide is employed.
Furthermore, a preferable mode is one wherein as the second capacity insulator film, a titanium oxide film having a film thickness of 0.2 to 1.0 nm is formed.
With the above configurations, it become possible to reduce the leakage current while suppressing a decrease in capacitance of the capacitor with a reduced film thickness calculated as silicon oxide film. That is, the leakage current can be suppressed while securing a capacitance large enough to permit the capacitor to operate normally.
Also, it become possible to manufacture the capacitor easily without requiring any special steps.